1. Field of the Invention
The present invention relates generally to the field of power transmission through gearing and, more specifically, to a cooling system for high power gears.
2. Description of the Related Art
When two gears mesh, heat is generated as work is transferred from one gear to the other. In many types of drive trains, the rotary speed of the gears is such that lubrication, and heat removal, can be effected by immersion of at least a portion of the gears in a supply of lubricant. This may be performed in a variety of known ways, including having a gear extend into a sump of lubricant, or having oil pumped from a sump and distributed over the gears.
Gearing that uses immersion-type lubrication generally relates to machinery having engine output shafts rotating at speeds from a few hundred rpms to a few thousand, and under these circumstances heat can be removed by simply having an exterior surface of the sump exposed to an airflow. Moreover, at these relatively slow rpms, the immersion lubricant does not significantly impede rotation of the gears themselves.
Special lubrication problems for relatively slow gears have resulted in gear structures that include passageways for communicating lubricant therethrough. For example, in U.S. Pat. No. 4,771,864, issued to Lorimor et al., lubricant is supplied to a spline joint by providing a radially inwardly facing chamber at one end of the joint and filling the chamber with lubricant. An internal barrier assures a fixed internal radius and thus a predetermined centrifugal pressure head, while a plurality of venting passages located radially outwardly of the radius continually exhausts fluid from the splines as a result of centrifugal force so that fresh fluid is circulated through the splines.
U.S. Pat. No. 2,681,126, issued to Searls, shows lubrication of compound gears whereby channels are formed through bearings on which the gears are mounted. The gears form part of a gear changing-type gear box, and the purpose of the channels is to put lubricant on the gear teeth.
In a similar fashion, U.S. Pat. No. 4,696,201, issued to Hattori et al., describes a gear assembly in which lubricant is supplied through the gear shaft and channeled radially outwardly to the teeth through internal passages.
In high speed high power density gearing, such as the gearing found in aircraft gas turbine engines, the speed of the gears makes it unsuitable to use immersion-type lubrication. On the other hand, the high speed nature of the gears facilitates impingement-type lubrication whereby a small jet of lubrication is sprayed onto the gears, and the sprayed lubricant is collected and recirculated.
Heat becomes a more significant problem to a high speed gear box in that the heat generated where the gears mesh will be substantially greater due to the amount of power being transmitted. It is generally known to effect cooling by placing oil jets at the downstream side of the meshing zones of the gears so that the jetted oil is discharged directly at and impinges on the tooth flanks. Since the tooth-face temperature rise will be at its peak immediately after meshing, this location of the jet was considered to be best since most of the heat is at the surface of the teeth.
Typically, the oil is scavenged, after passing over the gear teeth, and then filtered and cooled before recycling through the gear box. In many gas turbine engines, the heated oil is placed in a heat exchanger in which the cooling fluid is the fuel immediately before it enters the combustion zone of the engine. Thus, the fuel is heated prior to combustion, and the oil is cooled by fuel.
Should a 50,000 horsepower jet engine experience a 1% loss to heat, 500 horsepower will be lost and 1,272,540 BTU/Hr will be generated, which must be disposed of. This loss will manifest itself in higher fuel consumption and a greater requirement to dissipate this energy. Additional cooling flow, which would be necessary for high power engines, will potentially give rise to additional heat losses, if introduced through normal lubrication jet systems. This is due to the fact that oil in excess of lubrication requirement will result in viscous losses.
In gear boxes associated with such engines, for example, in a high bypass ratio turbofan engine, it is very important to scavenge lubricant that is sprayed from the lubricant jets and remove it from the gear box as quickly as possible. Windage in the gear box, generated by motion of the gears at a very high speed, makes it difficult to scavenge oil. Moreover, gear box volume is extremely limited and thus scavenging problems become more severe. In other words, increasing the flow of oil through the lubricant jet system currently in place is not a desirable option for removing additional heat associated with high power gear boxes.